1. Field of the invention.
This invention relates to television systems.
In the type of television signal currently broadcast within the United Kingdom, there is a period between transmission of the last line of each picture frame and the beginning of the first line of the next frame in which no picture information is sent. This period has a duration approximately equal to that of twenty five lines. The first part of the period is taken up by a predetermined pulse waveform which includes frame synchronising pulses and the latter part (for a duration of eight lines) consists solely of line synchronising pulses, i.e., in this second part the waveform is the same as that during picture transmission except that there is no picture information between the line synchronising pulses. These blank lines can be considered as representing a margin at the top of the picture that is displayed on the screen of a conventional television receiver.
It has recently been proposed to utilise one or more of these blank lines in each frame to transmit selected items of data which can be displayed on the screen of a television receiver either in addition to or in place of the normal television picture. A system that is capable of handling both picture information and data that is transmitted in blank lines of the picture is hereinafter referred to as a "combined television/data system".
It is convenient to identify the data that can be displayed at any one time as a page by which term is subsequently referred to herein. The particular page displayed of those pages of data being transmitted is selected by operation of a suitable control of equipment forming part of or associated with the televison receiver.
Each line of data within a page is hereinafter referred to as a data line. In fact the display of each data line involves a plurality of line scans of the cathode ray tube or other display device of the television receiver. Combined television/data systems of the kind with which the present invention is concerned, rely on each character to be displayed being characterised in digital, preferably binary, form and accordingly the equipment mentioned in the last paragraph includes a plurality of character generators which are each arranged selectively to supply the appropriate modulating signals to the display device during a succession of line scans to cause the required character to be displayed.
In view of the limited amount of data that can be transmitted during each picture frame, it is necessary for the equipment forming part of or associated with a television receiver for the purpose of displaying data to have provision for storing at least the page of data that is required to be displayed. Such storage is periodically updated but, for this to be done satisfactorily, it is clearly essential for the equipment to be able to identify in some way the data received during each picture frame.
2. Description of Prior Art.
In the proposed arrangements previously referred to, it has been thought necessary for the address information transmitted with the required data in the otherwise blank lines to include the page identification in each picture frame. Thus in one arrangement, two adjacent blank lines in a frame are utilised to signal one data line, the first of these blank lines containing the identity (in digital form) of the page and data line and the first half of the character of the data line while the second blank line carries the characters of the second half of the data line. In another arrangement one blank line in each of four successive frames is utilised to signal each data line, each such blank line carrying details of one quarter of the characters that make up the data line prefaced by the identity of the page, the data line and the particular quarter of the data line.
It has now been realised that it is not necessary to effect such complete addressing of the data signalled in each picture frame and that, as a result, it is possible for a single blank scan line to be used for signalling a complete data line.